As the microelectronics industry moves into ultralarge scale integration (ULSI), enhancement in performance speeds of integrated circuits will be achieved by reducing the device feature size and thereby the overall die size. As a result, density constraints will require multilevel structures with vertical interconnects. It is expected that the use of metals with lower resistivity, such as gold, silver and especially copper, will be necessary because of the submicron geometries.
Fabrication of interconnect structures includes one or more metallization steps. Metallization is commonly accomplished by physical vapor deposition (PVD) processes, including evaporating and sputtering. Chemical vapor deposition (CVD) processes have an advantage over these so-called "line of sight" processes in the fabrication of submicron vertical interconnects because conformal layers of metals are more easily produced.
In CVD, a volatile precursor, usually a complex of a metal with an organic ligand, serves as a source of the metal. The precursor is delivered to the substrate in the vapor phase and decomposed on the surface to release the metal. The precursor must exhibit sufficient thermal stability to prevent premature degradation or contamination of the substrate and at the same time facilitate easy handling. Vapor pressure, the adsorption/desorption behaviour, the chemical reaction pathways, the decomposition temperature can directly affect the purity of the deposited metal film and the rate of thin-film formation.
CVD precursors very frequently are based on complexes of metals with .beta.-diketonates such as 2,2,6,6-tetramethyl-3,5-heptanedione (thd) and acetylacetonate (acac) and fluorinated .beta.-diketonates, such as 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (hfa or hfac) and 2,2-diethyl-6,6,7,7,8,8,8-heptafluoro-3,5-octanedione (fod). Volatility of the non-fluorinated precursors is insufficient for many applications. The fluorinated analogs possess greater volatility, but also have a tendency to fragment, a consequence of fluorine migration/carbon-fluorine bond cleavage at elevated temperatures, leading to contamination of the substrate. Consequently, a need exists for precursors which retain volatility yet release the metal without degradation of the ligand and for ligands which are not labile or disposed toward fragmentation.
It is therefore an object of this invention to develop metal complexes for CVD precursors that are highly volatile and yet stable at the sublimation point and also retain desirable processing features. It is a further object to develop ligands for use in CVD precursors which can induce high volatility in a metal and can release the metal without degradation of the ligand. It is a further object to provide new synthetic routes for the synthesis of these ligands from commercially available starting materials in good yields.